1. Field of the Invention
The present invention relates to a sheet feeding apparatus which can separate sheets one by one by utilizing the friction force of the sheets stacked in a cassette and then can feed it to an image forming system.
2. Related Background Art
An example of a conventional sheet feeding apparatus of this kind is shown in FIGS. 14 and 15.
In FIGS. 14 and 15, a cassette 101 has an intermediate plate 103 on which sheets are stacked and which is pivotally supported on pivot pins 105 at its base end. Upper limit pegs 102 for limiting a stacking amount of sheets to be stacked in the cassette 101 are formed on a rear wall of the cassette at its upper end. Further, a size indicating means 104 for transmitting the size of the sheet stacked in the cassette to the sheet feeding apparatus and an image forming system is provided on an outer surface of a side wall of the cassette 101. A pair of separating pawls or claws 106 are mounted for movement in an up-and-down direction at front corners of the cassette 101, and a base portion of each separating claw is pivotally mounted on a caulking pin 116. The separating claws 106 serve to urge against front corners of the sheet stack rested on the intermediate plate 103 and to separate an uppermost sheet from the other sheets.
A support plate 110 is disposed in the vicinity of the other side wall of the cassette 101, and a pressure spring 107 is fixed to an inner surface of the support plate by a screw 109. An edge regulating member (not shown) is opposed to the pressure spring 107, which regulating member serves to regulate a position of one lateral edge (parallel to a sheet feeding direction) of the sheet stack in the cassette 101. A sheet guide 112 for guiding the sheet fed from the cassette 101 is formed on the front portion of the cassette 101. Further, the intermediate plate 103 is provided with a slot 113 for receiving a sensor for detecting the presence/absence of the sheet, and a separating sheet 115 adhered thereto and adapted to prevent the double feed of the last sheet.
However, in the above-mentioned conventional sheet feeding apparatus, since the pressure spring 107 was used to regulate the lateral edge of the sheet stack, the following drawbacks arose:
(1) If the sheet stack is obliquely rested (referred to as "oblique stack" or "skew stack" hereinafter) in the cassette 101, the skew-feed of the sheet to be fed cannot be avoided.
(2) If a spring force of the pressure spring 107 is increased to avoid the drawback as mentioned in the above Item (1), when a thin sheet stack is stored in the cassette 101 for a long time, the lateral edges of the sheet stack will be deformed, thus causing the bending of the sheets and/or the poor transfer of a toner image onto the sheet.
In order to eliminate the above drawbacks, an improved technique has been proposed, as disclosed in the Japanese Utility Model Laid-Open Appln. No. 54-159583.
According to this technique, a coil spring is arranged between a movable side plate adapted to be contacted with the lateral edge of the sheet stack and a pivotable actuator so that when the sheet stack is rested on a bottom plate, the actuator is cocked to reduce a distance between the movable side plate and the actuator, thereby urging the movable side plate toward the sheet stack by the relatively strong force of the coil spring to align the lateral edges of the sheets with each other, and, when the bottom plate is lifted to feed the sheet, the actuator returns to a laid condition to increase the distance between the movable side plate and the actuator, thereby urging the movable side plate toward the sheet stack by the relatively weak force of the coil spring. In this way, when the sheets are replenished, the lateral edges of the sheets can be aligned with each other, and, when the sheet is fed, the urging force of the movable side plate is decreased, thus preventing poor sheet supply.
However, with this arrangement, when the sheets are replenished, it is necessary to set the sheets on the bottom plate in opposition to the strong urging force of the movable side plate, thus severely degrading the operability.
Further, since the urging forces are adjusted in two-stages by the single coil spring, the values of the urging forces are determined unconditionally, with the result that effective urging forces cannot be obtained. That is to say, if the urging force for aligning the sheets with each other is set to a greater value, the urging force acting on the sheets when the sheet is being fed will also become greater, thus easily causing poor sheet supply; whereas, if the urging force acting on the sheets when the sheet is fed is set to a smaller value, the urging force for aligning the sheets with each other will also become smaller, thus causing poor alignment of the sheets.